The present invention relates to a system for controlling a continuously variable transmission.
JP-A No. 62-187628 discloses a conventional system for controlling a continuously variable transmission. This system is constructed and arranged so that a target reduction ratio is achieved by a feed-back control of a shift actuator. Specifically, a target value of an input revolution speed or a reduction ratio of the transmission is determined based on a vehicle speed and an engine load, and the feed-back control is carried out in a manner to conform to a current value of the input revolution speed or the reduction ratio to this target value.
A problem encountered in such conventional system for controlling a continuously variable transmission is that, even during normal operation of a motor vehicle, it is difficult to always keep the current value corresponding to the target value.
Specifically, with a continuously variable V-belt transmission, the reduction ratio is determined by a balance between a tension of a V-belt and a hydraulic pressure which acts on a movable conical member of a pulley. The tension of the V-belt increases as an engine torque is larger, so that the hydraulic pressure on the movable conical member is also to be increased in accordance with a rise of the tension. In this event, the movable conical member may not be held in a predetermined position due to increased leakage from seal portions, resulting in occurrence of a deviation of the current reduction ratio to the maximum reduction ratio.
On the other hand, with a continuously variable traction roller transmission which is disclosed, for example, in JP-U No. 63-84451, a position of each roller support member is controlled by a hydraulic servo apparatus, thus achieving a shifting. Accordingly, the hydraulic pressure within the hydraulic servo apparatus for position control of the roller support member should be increased as the engine torque becomes larger. As a result, in a manner similar to the above-mentioned continuously variable V-belt transmission, the current reduction ratio deviates to the maximum reduction ratio as the engine torque becomes larger.